Beam It Down

At first, the solar energy relayed from space would be used only to provide the minimal electrical power needed to run the electronics of the receiving station on the ground-much the way that line current powers conventional telephones. Ultimately, the satellites would beam down larger amounts of power, which could provide the megawatts of electricity that would contribute substantially to powering a village or even a city.

Scaling up to higher power levels would be straightforward, entailing simply the deployment of a larger amount of solar-collecting area in space. Power would be transmitted through the infrastructure of transmitters and receivers that will then be in place for the satellite communications systems. In this regard, microwave transmission has a decided advantage over conventional cable methods of transmitting power. A microwave system that is 80 percent efficient at sending 1 kilowatt will still be 80 percent efficient at sending 1 megawatt. This is fundamentally different from an electric utility transmission line, where you need thicker, and costlier, wires to carry more power. If too much power is put through a cable, it will melt the insulation.

Some fear that a network of solar power satellites could turn the atmosphere into one big microwave oven, cooking whatever wanders into the beam’s path. In reality, the microwave intensities that we propose would be orders of magnitude below the threshold at which objects begin to heat up. People would be exposed to microwave levels comparable to those from microwave ovens and cellular phones. While some critics speculate that microwaves pose nonthermal threats to human health, there is no reliable epidemiological evidence for adverse effects from microwaves at these low levels. Higher levels of microwave radiation would be found at the rectennas on which the beams are focused, but fences and warning signs could demarcate these areas of possible danger. But according to our calculations, microwave intensities even at the perimeter of the rectenna would fall within the range now deemed safe by the Occupational Safety and Health Administration.

A bigger potential problem is that of sharing the limited frequencies in the microwave spectrum. Motorola has come under fire, for example, because its planned system will employ frequencies in the 1.616-to-1.626-gigahertz range, which almost overlaps the 1.612-gigahertz frequency that astrophysicists tune to when gathering data about the cosmos. Radio astronomers worry that interference from a solar power satellite will overwhelm the comparatively weak signals they are seeking to detect. Motorola promises to limit spillover of its communications beams into the radio astronomers’ frequency niche, but the issue underscores the fact that the microwave spectrum is a limited resource jealously guarded by commercial and nonprofit users alike. Allocation of the spectrum must be addressed promptly and effectively to avoid preemption of space power technology before it’s born.

Whether solar power satellites become a reality will ultimately depend on the willingness of telecommunications and electric utility companies to enter the space power business. So far, neither industry has shown much interest. But then, they are for the most part unaware of the commercial possibilities. One has to know that an option exists to choose it. Thirty years ago, communications satellites were a novelty. Ten years ago, no one had heard of the Internet.

What is certain is that the present push for deregulation has led to a scramble on the part of telecommunications, computer, cable TV, and utilities industries to enter each others’ markets. Some electric power companies want to enter the telecommunications business as a way of capitalizing on the huge investment in wire and cable that reaches virtually every building in the country. It makes equal sense to propose that communications companies enter the power business. In practice, consortiums of power and communications companies might develop the proposed technology together.

No single piece of this technology poses a fundamental stumbling block. The physics of photovoltaic cells and microwave generation are well understood. To move to the next stage, though, will require a demonstration that all the pieces of this system can work together: the solar panels, the phased-array microwave antennas, the receiving stations that separate the data signals from the power beams, and the computers that tell the satellites where on the ground to aim the beams. NASA could accelerate this development tremendously by placing into orbit a prototype of a solar power satellite.

The benefits are too large to walk away from. A network of solar power satellites such as what we propose could supply the earth with 10 to 30 trillion watts of electrical power-enough to satisfy the needs of the human race through the next century. Solar power satellites thus offer a vision in which energy production moves off the earth’s surface, allowing everyone to live on a “greener” planet. Consider the philosophical implications: no longer need humankind see itself trapped on spaceship earth with limited resources. We could tap the limitless resources of space, with the planet preserved as a priceless resource of biodiversity.